The present invention relates to television synchronization ("sync") circuits, and more particularly to locking such a sync circuit to an external reference signal ("genlock" circuit).
Present genlock circuits use a sync separator which separates sync from an external reference video signal, and derives therefrom a vertical reset pulse which rapidly resets a vertical counter of the sync pulse generator to be locked when the external video signal is first connected to the genlock circuit. This rapid resetting causes a transient phase disturbance in the vertical sync signal of the slaved sync source. Unfortunately, false triggering also is possible in the horizontal sync signal when improper sync separation is caused by a bad connection of the cable conveying the external video or other such temporary interruption of this signal. As a result serious picture disturbances occur. Also power transistors used in deflection circuits and switched mode power supplies could be destroyed by fast sync timing changes. Circuits are known, such as shown in U.S. Patent application Ser. No. 201,435 now U.S. Pat. No. 4,358,740, filed Oct. 28, 1980 in the name of K. Dinter, and assigned to the assignee of the present invention, which use fifty percent duty cycle signals and have horizontal or twice horizontal frequency signals at the output of a phase detector so that the free-running frequency of the slaved generator is the same as the locked frequency. This may require a complex low pass filter for adequate smoothing which filter causes losses and therefore, additional amplification may be needed. Further when genlocking, it is desirable to use the short path phase difference, e.g. -10.degree. C. rather than 350.degree., between the signals to still further reduce disturbances.
It is therefore desirable to have a genlock circuit that does not cause picture disturbances or damage to transistors when subjected to an intermittent external sync signal or when first connected to an external video signal and does not need complex filters or additional amplifiers.